Question: In chemistry class, the girl to boy ratio is $8$ to $4$. If there are $16$ boys, how many girls are there?
A ratio of $8$ girls to $4$ boys means that there is a group of $8$ girls for every group of $4$ boys. If there are $16$ boys, then there are $4$ groups of $4$ boys each. Because we know the ratio is $8:4$ , there must be $4$ groups of $8$ girls each. There is a total of $32$ girls in chemistry class.